Warped necks affect playing comfort, that may result in 'higher action' even though the action at the bridge has been lowered.
generally tightening the truss, brings the neck 'down' loosening the neck brings the neck 'up' but this may vary from guitar to guitar and also where the truss access is located.
Usually some curvature is needed (this is called 'relief') to compensate for the height of the strings at the bridge and reduce fret buzz
If your bass is a cheap guitar, you might want to do it yourself, so you can learn how to adjust a truss, and you can perform your own guitar maintainence in future, as necks might sometimes warp from time to time due to weather conditions, etc.
The trick is getting the correct alan key that fits the truss.
Good Luck!